The present invention concerns a photonic crystal structure for mode conversion.
The invention applies to the area of integrated optics.
In known art, the function of a mode converter or transformer is to convert a first optic mode which propagates in a first guided propagation structure into a second optic mode able to propagate in a second guided propagation structure. The two guided propagation structures may either form part of one same photonic integrated circuit, or form part of two separate circuits. A guided propagation structure may, for example, be an optic fibre.
FIG. 1 shows a guided propagation structure with vertical confinement of known art. Light propagates in direction z. In vertical direction y, the light is confined in a layer 1 of material of high index n1 sandwiched between two layers of material 2, 3 of lower index n2 and n3, respectively. In lateral direction x, the light is confined in the medium of index n1 either by a material charge (charged strip) or by another material (etched strip), this other material possibly being air. The materials of indices n1, n2, n3 may be semiconductor, dielectric or metallic materials. The size and shape of the propagation mode or modes borne by the structure in FIG. 1 are dependent upon the geometry of the structure and the value of indices n1, n2, n3.
Another propagation structure is shown in FIG. 2. FIG. 2 is an overhead view of a photonic crystal propagation structure.
The photonic crystal structure consists of a set of aligned patterns 4. Patterns 4 may for example be holes etched in a material, or pillars grouped side by side. In a one-dimensional application, the patterns may be parallel grooves etched in the material.
Different types of propagation guides may be made in a photonic crystal structure. A first type of guide consists of a photonic crystal structure in which n rows of neighbouring patterns are missing. In the remainder of the description, this first type of guide shall be called “Wn guide”. By way of a non-restrictive example, FIG. 2 shows a structure in which a single row of patterns is missing. The guide is then a W1 guide. Wave propagation is based on the zone devoid of patterns (zone of width d in FIG. 2). Other types of guide are also known such as, for example, photonic crystal structures in which the zone devoid of rows of patterns does not correspond to the absence of an integer number of rows but to the absence of a non-integer number of rows (Wn guides with n real number), or photonic crystal structures in which patterns of different shape and/or size are made, for example holes of different sizes.
A guided propagation structure is optimised to best ensure a given function. It is the geometry of the structure which determines the propagation mode or modes. Mode conversion is very often necessary to pass from a first propagation structure ensuring a first function to a second propagation structure ensuring another function. Ideally; mode conversion makes it is possible to couple almost all the light getting out of the first structure into the second structure, to render negligible both reflection and losses through wave diffusion at the interface between the structures, and to ease alignment tolerances between the functions quite considerably, said alignment often accounting for high fabrication costs.
Examples of mode converters of known art are shown in FIGS. 3, 4 and 5.
The mode converter shown in FIG. 3 is disclosed in the document titled “Spot Size Converter for Low Cost PICs” (K. D. Mesel, I. Moerman, R. Baets, B. Dhoedt, P. Vandaela and J. Stulemeijer, ECIO'92, ThC2).
The converter in FIG. 3 is a converter of lateral type. A medium 5 of index n1 is sandwiched between two media 6 and 7 of respective indices n2 and n3. Mode conversion is achieved by the continuous widening of medium 5 along the direction of wave propagation. Widening of the propagation medium making it possible to pass from a first to a second propagation mode has to be made over a sufficiently long distance to achieve good performance. As an example, distance 1 over which widening is made may lie between 100 μm to 200 μm. The range of the wavelengths concerned is then the near-infrared (wavelengths of between 0.8 μm and 2 μm).
A second example of mode converter of known art is shown in FIG. 4. This second example of converter is disclosed in the document titled “Tapered Couplers for Efficient Interfacing Between Dielectric and Phototonic Crystal Waveguides” (Attila Mekis and J. D. Joannopoulos; Journal of Lightwave Technology, vol. 19, No 6, June 2001).
The converter in FIG. 4 makes it possible to pass from a propagation mode in a dielectric guide of strip type to a propagation mode in a photonic crystal guide. The strip guide of width W enters the photonic crystal structure over a distance “a” and then tapers to a point over a distance “b”.
A third example of mode converter of known art is shown in FIG. 5. This third example of converter is disclosed in the document titled “Observation of light propagation in two-dimensional photonic crystal-based bent optical waveguides” (S. Yamada et al.; Journal of Applied Physics, vol. 89, No 2, 15 Jan. 2001).
The converter in FIG. 5 allows transition between a two-dimensional waveguide and a photonic crystal guide. Here transition is assured by the funnel shape of the region devoid of patterning in the photonic crystal guide. Good matching is only possible if the funnel-shaped region is sufficiently long.
As a general rule, it appears that the mode converters of known art need to be fabricated over sufficiently long lengths in the direction of wave propagation in order to be able to operate properly. These long distances have disadvantages. They are detrimental for example for circuit fabrication, in particular circuits which have to reach good dynamic performance levels. For example, to modulate a laser directly at 10 Gb/s it is preferable that its length should not be more than 100 μm. It is therefore not possible to equip such laser with a mode converter of known art. Also a non-negligible quantity of material is needed to fabricate these converters, which leads to a substantial increase in size and hence the costs of components.
The invention does not have the aforesaid disadvantages.